1. Field of the Invention
This invention relates to pulse-width modulation (PWM) systems in general, and in particular this invention relates to PWM amplifiers and linear amplifiers using PWM power supplies.
2. Description of Related Art
FIG. 1 illustrates the concept of pulse-width modulation (PWM) using a functional diagram of a conventional PWM amplifier system 10. An analog waveform 15, a sine wave for example, is provided to an input of a PWM amplifier 20. A second input of PWM amplifier 20 receives a periodic signal 30 from an oscillator.
PWM amplifier 20 combines waveform 15 and periodic signal 30 to create a PWM waveform 40 that switches between two voltage levels (e.g., zero and one hundred volts). The durations of the pulses, or positive "swings," of PWM waveform 40 are selected so that the integrated energy of PWM waveform 40 equals the energy of analog waveform 15 multiplied by a selected gain factor. The gain factor is the gain of PWM amplifier 20.
A filter 60 filters PWM waveform 40 to produce an analog waveform 50, a replication of analog waveform 15 multiplied by the gain factor. The filter function is typically accomplished using a mechanical or electrical filter, such as an electrical motor, that is too slow to respond to the square-wave modulation frequency of periodic signal 30.
PWM amplifier systems are typically noisier, less linear, more complex, and exhibit higher distortion than equivalent analog amplifier systems (e.g., conventional analog amplifiers). Despite these shortcomings, PWM amplifier systems are widely used because they offer superior efficiency. Amplifier efficiency can be approximated by multiplying the difference between the input voltage and the output voltage (i.e., Vout-Vin) by the output current Iout. The voltage difference Vout-Vin can be substantial in analog amplifier systems, especially in high-power applications. In contrast, there is virtually no difference between the input voltage and the output voltage Vout of a PWM amplifier, except during the switching interval when Vout slews between voltage levels. Faster switching speeds reduces the switching interval and therefore improves efficiency. It is not uncommon, for example, to obtain energy transfer efficiencies as high as 90% to 98% in PWM amplifier systems. In contrast, the efficiencies of comparable analog system may be 25% or lower.
High-power PWM amplifiers require that the difference between the input and output voltages (Vout-Vin) be relatively large. Unfortunately, as the voltage difference increases, so too does the difficulty of precisely controlling small-valued output signals. This is because small valued PWM output signals require on times (pulse widths) that are short relative to the total PWM period. Consequently, timing errors, non-linear switching characteristics, and other types of distortion are large relative to the pulse widths for high-power PWM amplifiers operating at low power.
FIG. 2 illustrates the difficulty of precisely controlling small-valued output signals using high-power PWM amplifiers. FIG. 2 compares an ideal voltage waveform (dashed line) with a waveform distorted as a result of a non-ideal switching time .tau.. In the first example the ideal energy E is equal to the voltage swing (50V) multiplied by the pulse duration 6 T, or E=300 VT. Also in the first example, the non-ideal energy E' is somewhat less than the ideal energy E due to the switching time .tau.. However, the resulting error is relatively small due to the switching time .tau. being short relative to the pulse width.
The error is far greater in the second example, which illustrates the operation of a PWM system at lower power. Low-power output signals are created by reducing the pulse width: the switching time .tau. remains the same. Thus, the switching time .tau. is large relative to the short pulse width of the second example. The non-ideal energy E' is less than 30% of the ideal energy E. This significant reduction in accuracy at low power is further emphasized by other types of signal distortion, such as noise spikes and ringing. There is therefore a need for a high-power PWM system with improved low-power performance.